1. Field of the Invention
The present invention relates to a technique for bonding two members by using micro metal particles (metal nano particles).
2. Description of the Background Art
As a conventional bonding method, particularly, a method for bonding a semiconductor to an insulating substrate, a soldering material has been used. However, a soldering material has a low heat resistance and decreases reliability in a temperature of around 200° C.
Thus, as described in Japanese Patent Application Laid-Open No. 2004-107728, a technique for bonding two members by applying heat to metal nano particles instead of a soldering material is proposed. In this bonding method, composite metal nano particles in which metal particles having an average diameter of approximately 100 nm or less are bound and coated by an organic matter are a main component of a bonding material. In the bonding method for bonding two members, a metal nanopaste, in which the composite metal nano particles are dispersed in an organic solvent, is interposed between bonding parts of the two members which are at least heated to vaporize an organic component.
A silver nanopaste including silver nano particles coated with an organic solvent is used to change bonding conditions, such as temperature, time, and a pressurizing force, in order to bond copper to each other, and then the bonding strength is measured and a cross sectional structure of bonding is observed, which are described in “Bonding Process Using Silver Nano Particles: Study of Bonding Properties to Cu” by Eiichi Ide, Shinji Angata, Akio Hirose, and Kojiro F. Kobayashi. Proc. 10th Sympo. on Microjoining and Assembly Technologies for Electronics, 2004, Yokohama Japan, (2004) 213.
A bonding principle of the metal nano particles will be described. In general, as a diameter of particles is decreased, the particles have more active surface state than that of bulk metal. Thus, the particles are mutually sintered to grow, and the reaction easily proceeds to reduce a surface energy. This phenomenon occurs especially in nano particles (generally, particles have a diameter of 100 nm or less), as described in “Ph. Buffat and J-P Borel, Phys. Rev. A 13 (6) 2287 (1976)”.
By exploiting this phenomenon, it is capable of bonding at far lower temperature than a melting point in a bulk state, and furthermore, a bonding layer after bonding is achieved will not melt again up to the melting point of bulk. A surface of particles actually used for bonding is protected by an organic protective film to suppress sintering, and the protective film is decomposed and degassed by heating during bonding, to thereby achieve bonding.
To easily use the metal nano particles with the organic protective film as mentioned above and to prevent them from oxidation during decomposition, they are dispersed in the organic solvent to reduce viscosity. The solvent including the metal is referred to as a metal nanopaste.
Japanese Patent Application Laid-Open No. 2006-202586 proposes a bonding method which allows for an increase in a bonding area by forming a concave portion in a bonding part of one of members and forming a convex portion in a bonding part of the other member.
The bonding method using sinterability of metal nano particles needs pressurization to obtain high reliability, which is described in Japanese Patent Application Laid-Open No. 2004-107728 and Ide et al., 2004. To perform pressurization, members having the metal nano particles interposed therebetween need to resist pressurization. However, when the members cannot resist pressurizing force, a chip crack and a substrate crack may occur in a semiconductor and an insulating substrate.
To reduce pressurizing force, it is conceivable that the bonding method in Japanese Patent Application Laid-Open No. 2006-202586 is employed. However, in this bonding method, when the metal nano particles do not reach to all of the concave portions in the bonding parts of the members, a bonding strength and a bonding reliability may be decreased.